Steel bar heater

ABSTRACT

A machine for electrically heating steel bars includes a laterally oscillating bar feeder for feeding steel bars at randomly disposed positions to clamping electrodes which clampingly engage the ends of the bars while electric current is supplied thereto through a low inductance circuit and which electrodes are spring loaded to accommodate longitudinal expansion of the bars during heating and to break said bars loose of the electrodes after heating.

United States Patent [191 Gray [451 Apr. 23, 1974 STEEL BAR HEATER [76] Inventor: Donald 1. Gray, P.O. Circle Dr., St. Charles, 111. 60874 [22] Filed: Feb. 12, 1973 [21] Appl. No.: 331,622

[52] US. Cl. 219/156, 219/50 [51] Int. Cl C21d 9/62 [58] Field of Search 219/156, 50, 56,158,161,

[5 6] References Cited UNITED STATES PATENTS 3,705,973 12/1972 Balzer et al 219/156 3,548,149 12/1970 Rizzolo et al. 219/156 3,418,447 12/1968 Rizzolo et al. 219/156 2,931,892 4/1960 Stitch et a1 219/156 Primary Examiner-Bruce A. Reynolds Attorney, Agent, or Firm-Edmond T. Patnaude 5 7 ABSTRACT A machine for electrically heating steel bars includes a laterally oscillating bar feeder for feeding steel bars at randomly disposed positions to clamping electrodes which clampingly engage the ends of the bars while electric current is supplied thereto through a low inductance circuit and which electrodes are spring loaded to accommodate longitudinal expansion of the bars during heating and to break said bars loose of the electrodes after heating.

10 Claims,' 8 Drawing Figures PATENIEDIIII 23. I974 3 8 O6, 697 SHEET .5 [IF 5 TIME-SECONDS ITEM DESCRIPTION 0 IO 20 3,0 40 45,46 WELDING TRANSFORMERS WITH TAPPED PRIMARIES II? II6 II6(1 IIGD CHOKE, 2 MILLIHENRY 480V, 3 AMP FUSES CONTROL TRANSFORMER, 48OV-I2OV IIEIME DEPENDANT SEQUENCER. I5 SECS. PER COMPLETE REVOLUTION HOLDING CONTACT FOR CAM MOTOR BAR FEED CONTACT BAR CLAMP CONTACT SHORT DELAY CONTACT LONG DELAY CONTACT l2) N RESET TIMER MANUAL RuN,ACTuATE FOR I MACHINE CYCLE TIMING PER'ODI MOTOR wHICH DRIvEs 7 CIRCuIT CAM TIMER CAM MOTOR SERIES CONTACT MANUAL, ACTuATE TO ENERGIZE CONTROL I POwER I EMERGENCY STOP, ALL CONTROL POWER OFF RELAY COIL, CONTROL POWER RELAY HOLDING CONTACT CONTROL CIRCUIT ENERGIZE D.C. MOTOR IN RESET TIMER USED FOR I CONTROLLING MAXIMUM BAR TEMP.

M g/ I ACTLIATE FOR CONTINUOUS MACHINE RELAY COIL, CoNTINuOus CYCLING CONTINUOUS CYCLING NOT APPROPIATE I FOR THIs CHART AIR SOLENOID FOR BAR FEED MANUAL, ACTuATE TO FEED BARS, USED FOR MACHINE sET-uP AIR sOLENOID FOR BAR CLAMP, Low PREssuRE F MANuAL, ACTuATE TO CLAMP DARs, USED FOR MACHINE SET-UP RELAY COIL =I H RRI IE B R EAT P0 R RESET TIMER CLuTCH REsET TIMER CONTACT REsET TIMER CONTACT HEAT POWER CONTACTOR POWER TRANSFORMER CONTACT POwER TRANSFORMER CONTACT M ggAmgIgmT FggWESpON-OFF swITCH, USED I SECOND DELAY RELAY COIL HIGH PREssuRE AIR sOLENOID CONTAC MANuAL. SELECT swITCH FOR EITHER 3 sECOND (I2Ie) POWER CONTACTOR, CHOKE BY-PAss CHOKE BY-PASS CONTACT 2 AIR SOLENOID FOR BAR CLAMP, HIGH PRESSURE Mggggfigfi VARYING BAR-CONTACT TIP BAR TO BE HEATED BAR CONTACTS 1 STEEL BAR HEATER The present invention relates in general to apparatus for heating steel bars by passing electric current directly therethrough, and it relates more particularly to an automatic machine for electrically heating steel bars prior to hot forming thereof as by forging.

BACKGROUND OF THE INVENTION Over the years, various methods have been developed for heating steel bars for subsequent forming into more complex shapes, principally by the forging process, but the heating method most often employed utilizes a gas or oil fired furnace. This heating method is well accepted by the forging industry because of the relatively low initial investment per heated ton of steel, the low level of maintenance skill necessary for occasional furnace repairs, and the ability to heat nearly any length or shape of bar desired.

There are, however, several disadvantages to this method, including the following: a heavy crusty scale is formed on the surface of the bar, which interferes with the quality of the forged part and increases the wear rate of the forming dies; start-up and shut-down of these furnaces is time consuming and can accelerate thermal cycling failure of the furnace; much of the available energy from the burning fuel is wasted to the surroundings; operator discomfort results from both the wasted heat and the fumes; and the bar temperature, to a large degree depends on the skill of the furnace operator.

Within the last decade several electric bar heating methods have become accepted for certain specialized bar heating applications. These methods include resistance, and induction heating. Specifically, induction heating has been utilized to heat bar lengths of 8 inches or less, and also to heat large bars when the heated tonnage exceeds 3,000 pounds per hour. This method has the principal disadvantages of requiring a very substantial initial capital investment and highly skilled operators and maintenance personnel. The initial cost of an induction heating installation is from ten to fifty times as expensive as a fuel fired furnace installation and quite complicated in both theory and practice. Thus, induction heating is usually used only in manufacturing plants that have engineering expertise, and a skilled maintenance crew.

The electric resistance bar heating method, in contrast to the induction method of heating, is characterized as a contact heating system wherein the electric energy is transferred directly into the bar of steel by one or more sets of contacts between which the bar is held. As compared to other methods of bar heating, the electric resistance method has a number of advantages, including lack of appreciable bar scale because of the rapid speed at which the bar is heated; ease of start-up and shut-down; ease with which job changes can be accomplished; no operator discomfort due to heat or fumes; and little machine maintenance. While the maintenance is more complex than is required for a fuel furnace, it is, nevertheless, well within the capabilities of the average shop.

Among the disadvantages of the electric resistance heater are the following: bar lengths must be pre-cut to forging length plus an additional length must be included to act as a handle, which additional handle length of steel is a serious waste disadvantage when making short forgings, although it is considered less serious when making long forgings; the bars must be cleaned of rust and scale to enhance the electrical contact (this is accomplished most often by shot blast cleaning); the bars must be sheared rather accurately to provide consistent ends for the electric contact; insufficient heating capacity per machine hour (practical limits of typical prior art machines range front 200 to 600 pounds of 12 inches long burs per machine hour; and controlling the desired bar temperature when using resistance bar heaters is inherently difficult. The simplest temperaure control known in the prior art employs an interval timer to fix the length of time the bar is heated. Thus, a longer heating time yields a hotter bar. This method proves inadequate for most applications because of uncontrolled line voltage variations which result in uncontrolled variations in the bar temperature.

This relation is shown in simplified form below:

E varies as V and H E (E /R) X t and T E H Therefore T 5 (E /R) X t where V is the line voltage E is the voltage across the bar H is the heat input to the bar R is the bar resistance T is the maximum bar temperature I is the heating time interval This equation shows that small changes in the voltage across the bar produce large changes in the temperature developed in the bar. Because of this sensitivity of the bar temperature to line voltage variations, maximum bar temperature controls using timers have usually been unsatisfactory.

Another problem that is inherent to electric resistance bar heating is the tendency for the contact tips to stick to the heated bar so that the bar is not released from the contact tips at the end of the heating cycle. In addition, if the bar is not kept under tension during the heating cycle, the growth in length of the bar due to heat expansion will compress the bar and cause it to bend. Both of these problems interfere with subsequent hot forming of the bar. The solution employed in prior art machines uses an air cylinder to knock off the bar at the end of the heating cycle. A common method used to prevent bar bending is to apply still another air cylinder to the contact tip structure which applies a tension force to the bar. While these are an effective solution to the two problems discussed, the disadvantage of requiring additional equipment decreases overall machine reliability and increases the manufacturing cost of the machine.

Contact tip wear is another problem associated with resistance bar heaters. Developing sufficient force on the contacts to facilitate the passage of current into the bar during heating requires a high force between the contact tips and the bar. Because a cold bar has sharp edges remaining from the bar shearing operation, the pressure between the bar and the contacts can be enormous after clamping and prior to heating. Thus, hight pressure accentuates gouging of the relatively soft contact tips, which are usually made from copper or a copper alloy. Also, when current is first passed through the bar contacts, sparking often occurs which evaporates metal from the contact tips and causes erosion to occur.

SUMMARY OF THE INVENTION THe apparatus of the present invention overcomes many of the disadvantages described above while retaining the advantages of the electric resistance heating method by increasing the contact areas between the electrodes and the bars to reduce heating time and thus increase the tonnage per hour'of the machine; by automatically varying the heating period in response to changes in the RMS value of the line voltage; by using a variable electrode clamping force which is initially small to reduce damage to the contact electrodes and then is increased to facilitate the flow of current through the bar; by feeding the bars at purposely random positions to the contact electrodes to utilize substantially all of the electrode contact faces rather than the same small facial areas for every bar; and by a spring loaded clamping electrode assembly including a spring which permits movement of the electrodes as the bars expand during heating and utilizes this stored spring force to break each bar free of the electrodes when the bars are to be released upon completion of the heating cycle.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages and a better understanding of the invention may be had from the following detailed description taken in connection with the accompanying drawings, wherein:

FIG. 1 is a partially sectioned elevational view of a bar heating machineembodying the present invention;

FIG. 2 is a top view of the machine of FIG. 1 taken from the line 22 thereof;

FIG. 3 is a sectional view of the machine of FIG. 1, taken along the line 3-3 thereof and particularly showing the clamping electrode mechanism;

FIG. 4 is a vertical cross section of the bar feed mechanism;

FIG. 5 is a fragmentary sectional view taken along the line 5-5 of FIG. 1;

FIG. 6 is a perspective view of the machine showing the bar feed mechanism in an elevated position;

FIG. 7 is a schematic diagram of the control circuit for the machine of FIG. 1; and

FIG. 8 is a timing chart illustrating the sequence in which the electric control elements operate.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings and particularly to FIGS. 1, 2 and 6 thereof, a bar heating machine 10 includes a frame having a pair of upper members 12 and 13 supported on a plurality of uprights 14. A pair of mounting plates 18 and 19 are secured as by bolts to the forward end portions of the frame members 12 and 13 and a pair of support rods 20 and 21 are secured to and extend between the plates 18 and 19. Referring also to FIG. 3, on the rods 20 and 21 are mounted two spring loaded clamp and contact assemblies 24 and 25. These two assemblies include respective contact electrodes 26 and 27 which engage the ends of the bar to be heated. As described more fully hereinafter, the assemblies 24 and 25 are adjustably movable along the rods 20 and 21 to accommodate different bar lengths.

At the rear of the machine a rod 28 is supported between a pair of mounting plates 29 and 30 secured to the frame members 12 and 13 as by bolts, and two spring loaded clamp and electrode assemblies 32 and 33 are connected thereto through respective piston and air cylinders 34 and 35. The assemblies 32 and 33 include contact electrodes 36 and 37 disposed opposite to the electrodes 26 and 27 whereby the electrodes 26 and 36 provide one pair of electrodes and the electrodes 27 and 37 provide a second pair. The assemblies 32 and 33 are further supported by means of a metal strap 38 mounted at ends by a pair of swinging arms 39 to the frame members 12 and 13. As in the case of the clamp and electrode assemblies 24 and 25, the assemblies 32 and 33 are adjustably movable along the rod 28 and strap 38 for use with different bar lengths.

A plurality of electric power busses 41, 42, 43 and 44 are connected to the respective contact electrodes 37, 27, 26 and 36 and are electrically connected to the secondary windings of a pair of transformers 45 and 46, each secondary winding being connected to one contact electrode in each pair. As best shown in FIG. 3, the adjacent ends of the busses 41 and 44 are spaced as close as possible without arcing, and a pair of cables 47 and 48 are respectively connected between the inner ends of the busses 41 and 44 and the secondary terminals of the transformer 45. The cables 47 and 48 are insulated and lie in substantial mutual engagement between the transformer terminals and the busses. A similar pair of cables 49 and 50 are connected in like manner between theinner ends of the busses 42 and 43 and the terminals of the transformer 46. This arrange- 1 ment of the power busses and the power leads thereto provides a generally T-shaped power loop to a bar 51 held between the contact electrodes thereby increasing the efficiency of the system by minimizing the inductance in the system which reduces the size and cost of many of the components and also reduces the time required to heat the bar 51 to the desired temperature.

The four clamping electrode assemblies are basically the same, and therefore, only the assembly 25 is described in detail herein. As best shown in FIG. 3, the assembly 25 includes a pair of body members or blocks 60 and 61 connected together at one side by a hinge plate 62 fixedly connected to the body member 60 by a pair of bolts 63 and to the member 61 by a single bolt 64 about which the plate 62 pivots. The blocks 60 in each assembly are formed of a suitable insulating material such as Bakelite and the blocks 61 may be formed of metal. A coil spring 65 is mounted with its ends in oppositely located, aligned recesses 66 and 67 in the members 60 and 61 to bias them into the illustrated mutually aligned relative positions. The block 61 is drilled to provide a transverse hold to permit the block 61 to be moved along the rod 20 to adjust for different bar lengths. A forwardly extending lug 68 on the block 61 is also provided with a transverse hole which slidably receives the rod 21. A set screw is provided for locking the clamp and electrode assembly at the adjusted position along the rods 20 and 21. i

The contact electrodes are mounted to the blocks 60 by a pair of bolts 69 extending through respective ones of a pair of horizontal slots 69a and 69b in the buss bars 41-44. As best shown in FIG. 3- the contact faces are offset to the outside of the machine from the pivot bolt 64 so that as the contacts are forced into clamping engagement with the edges of the bar an outward force is exerted on the contact electrodes to maintain the bar 51 under tension during the heating operation. The faces of the contact electrodes are formed at a bias angle of between about 2 to 12.

OPERATION OF CLAMP ASSEMBLIES In operation, the air cylinders 34 and 35 are actuated to push the clamp and electrode assemblies 32 and 33 toward the assemblies 24 and 25 to clamp a bar 51 between the opposing faces of the contact electrodes as shown in FIG. 3 to hold the bar under tension. As the length of the bar 51 increases during heating thereof, the body member 60 and the corresponding body members of the other clamp assemblies pivot outwardly compressing the springs 65. Upon completion of the heating operation, the cylinders 34 and 35 are actuated to retract the electrodes 36 and 37 away from the electrodes 26 and 27. During such movement the inner body members 60 pivot inwardly under the force exerted thereon by the associated springs 65 to cause the four contact electrode faces to slide across the adjacent edges of the bar 51 to break the bar loose from the electrodes to which it tends to stick. The bar 51 then drops by gravity onto a continuously moving conveyor 53 which carries it to the forging station.

As best shown in FIGS. 1, 4 and 6, a bar feed mechanism 70 is mounted at the top of the machine to feed the bars 51 one by one to the clamping electrodes. In order to facilitate machine maintenance, the feed mechanism 70 is pivotally supported on the rod 28 so that it can be elevated to an upright maintenance position shown in FIG. 6 or lowered to the operative position shown in FIG. 1 where it rests at the front end on the frame members 12 and 13. The feed mechanism 70 includes an open frame to which a pair of mounting plates 71 and 72 are secured and between which a pair of rods 73 and 74 are supported. A bar magazine includes a pair of parallel arms 75 and 76 to which a pair of guide plates 78 are adjustably connected for accommodating different sizes of bars. A pair of angle irons 79 are respectively welded to the plates 78 such that one flange 80 of each angle iron provides a ledge for slidably supporting one end of each bar in the magazine. A plurality of slots 81 are provided in the arms 75 and 76 respectively to receive a plurality of bolts 82 for adjustably attaching the plates 78 to the arms 75 and 76.

As best shown in FIG. 4, the end portions 84 of the ledges 80 are curved downwardly and terminate at an adjustable distance above a shelf 85 onto which the bars 51 fall under the force of gravity. The slots 81 extend at an angle of about 45 to the plate 85 whereby adjustment of the plates 78 along the slots moves the lower edge of the ledges 80 equal vertical and horizontal amounts for use with different sizes of rounded corner square bars. It will be recognized by those skilled in the art that the machine may also be used with the usual cylindrical bars.

The bars 51 are fed from the shelf 85 to the clamping electrodes by means of a pusher plate 89 which is reciprocably driven across the shelf 85 by a bar feed pneumatic cylinder 89 mounted to the frame of the bar feed mechanism. As the plate 88 moves to the right as shown in FIG. 4, the lowermost bar 51 is pushed off the end of the shelf 85and falls between the electrode pairs 27 and 37 at one side and 26 and 36 at the other side. A pair of support arms 91 each in the form of a sandwich of leaves of steel are mounted on brackets which depend from the pusher plate 88 through slots 92 in the plate and the arms 91 extend forwardly of the shelf 85 to hold the bars between the electrodes after they drop from the shelf and before the cylinders 34 and 35 are actuated to clamp the bars 51. The leaves are preferably formed of a mild steel to prevent the bars from bouncing thereon when they fall from the plate 85. Mounted below the electrodes is the conveyor assembly 53 including a chain belt 95 which carries the heated bars out of the machine for transfer to the forging station. Preferably, the belt 95 is driven continuously while the machinel0 is operating.

As may best be seen in FIG. 3, the contact electrodes 26, 27, 36 and 37 have relatively wide, angled faces which engage the edges at the ends of the bars 51, and in accordance with one aspect of the present invention the bars 51 are dropped between the electrodes at randomly different axial positions whereby to use substantially all of the facial areas of the electrodes. To this end as best shown in FIGS. 2 and 5, a motor is mounted to the upper frame member 13 and the shaft 101 thereof is connected by a crank 102 and a crank arm 103 to the magazine 70 to reciprocate the magazine back and forth on two pairs of rollers 108 which roll on the frames 12 and 13. The crank arm 103 is apertured near its ends to fit over a pin 104 on the crank 102 and over a pin 105 on a guide block 106 fastened to the magazine 70, so as to be readily removable when it is desired to lift the magazine into the raised, maintenance position. One pair of the rollers 108 is journaled in the block 106 and rolls back and forth on the top surface of the frame member 13 as the motor shaft 101 rotates. The magazine is thus reciprocated relative to the stationary frame 13 through a distance slightly less than the width of the electrodes whereby randomly different facial areas of the electrodes engage the end edges of the bars.

OPERATION Refer now to FIGS. 7 and 8 in conjunction with which a typical operating cycle of the machine will be described. With the main input terminals 110 and 111 connected across a 440-volt AC source, the control circuits mounted in a control box 112 at the rear of the machine (FIG. l) are energized through a step-down transformer 113 whose primary winding is connected through suitable fuses to the power terminals 110 and 111. The machine is set into operation by momentarily closing a spring biased normally open start switch 115 to pick up a relay 116 having its solenoid connected in series with the start switch 115 and a normally closed stop switch 117 across the secondary winding of the transformer 113. A set of normally open holding contacts 116a on the relay 116 are connected in parallel with the switch 115 and thus hold the relay 116 picked up until the stop switch 1 17 is manually opened. AS shown, when the relay 116 picks up, a set of normally open contacts ll6b thereof, are closed to connect the secondary winding of the transformer 113 across a pair of control circuit power busses 118 thereby to energize the'control circuits. Closing of the contacts 1l6b thus energizes a DC timer motor 119 which, as more fully described hereinafter; controls the heating cycle of the machine to provide a constant bar temperature irrespective of line voltage variations. The motor 119 continues to run so long as the contacts 1 16b remain closed, i.e., until the stop switch 117 is actuated.

In order to initiate a bar heating operation wherein one bar is fed from the hopper, clamped between the electrodes, heated and then dropped onto the conveyor 53, a normally open switch 119 is manually closed and then a spring biased, normally open switch 120 is mementarily closed to energize a timer motor 121 through a set of normally closed contacts 122a on a relay 122 having its coil serially connected between the busses 118 with a set of normally open contacts 123a of a clutch operated reset timer driven by the motor 116 when the reset timer clutch 123 is energized. The timer clutch 123 is serially connected between the busses 118 with a set of contacts 121d controlled by the timer motor 121. When the motor 121 is energized it initially closes a set of holding contacts 121a which remain closed until the motor 121 has timed out one complete cycle. After the contacts 121a close, a set of contacts 1211) close to energize a solenoid 124 which opens an air valve (not shown) to actuate the bar feed air cylinder 89. Operation of the air cylinder 89 pushes the lowermost bar 51 across the plate 85 whereby it drops between the clamping electrodes onto the spring fingers 91. As shown, a manual switch 125 is connected in parallel with the contacts 121b to permit manual control for semi-automatic feeding of the bars during initial set up and maintenance of the machine.

With a bar 51 thus positioned between the clamping electrodes, timer motor contacts 121c close to energize an air valve solenoid 127 which opens a valve (not shown) to supply air pressure at a relatively low pressure to the clamping solenoids 34 and 35. The clamping electrodes 26, 27, 36 and 37 thus clampingly engage the ends of the bar 51 at a force of about 500 pounds.

At about this same time, timer motor contacts 121d close to actuate the reset timer clutch solenoid 123 to cause the timer motor 121 to close the set of contacts 123a to pick up relay 122. When the relay 122 picks up, a normally closed set of contacts 122a thereof serially connected with the timer motor 121 open. At the same time a set of normally open contacts 122b close to pick up a power relay 127 which closes a pair of power contacts 127a and 12711 serially connected with the primary windings of the transformers 45 and 46 between the power input terminals 110 and 111. Current thus flows through the bar 51 which is connected directly across the secondary windings of the transformers 45 and 46. In addition, when the relay 122 picks up and the contacts l22b close, the coil 128 of a time delay relay is energized and after a time delay of about one to two seconds closes it normally open contacts 128a to energize the solenoid 129 of a high pressure air inlet valve thereby to increase the pressure to the clamping cylinders 34 and 35 to cause the electrodes to clampingly engage the bar 51 at a force about two to ten times that of the initial clamping force. During the one-second time delay the bar is partially heated and softened whereby the subsequent application of the high clamping pressure minimizes the damage to the relatively soft copper electrodes.

The timer motor contacts 121e and 121f then closes in sequence to energize the coil 131 of a relay having a set of normally open contacts 131a connected in parallel with a current limiting choke coil 130 connected in series with the primary windings of the transformers.

A set of timer motor contacts 121g then open to deenergize the timer motor 121 whereby the machine is under the sole control of the timer motor 119.

When the reset timer 123 times out the preset heating time, its contacts 123a open to drop out relay 122. Contacts 122a thus close to restart the timer motor 121 and contacts 122b open to de-energize the relays 127 and 128. Power contacts 127a and 127b thus open to terminate the heating cycle and the clamping pressure is lowered by opening of the contacts 1280 in series with the high pressure air control solenoid 129.

The cam sequencer on the timer motor 121 now takes over and runs for a brief period of about one second to open the contacts 121a, c, e and f and to close the contacts 121g. When contacts 121s open, the low pressure air control solenoid 127 is de-energized. The springs 65 in the clamping electrode assemblies which were compressed as the bar 51 expanded during the heating cycle now expand to pivot the blocks 60 into alignment with the blocks 61 whereby the faces of the electrodes are moved across the adjacent edges of the bar 51 to release the bar 51 and permit it to drop by gravity onto the conveyor 53. The motor 121 then stops and the cycle is completed.

In order to provide continuous recycling of the machine, a switch 131 may be closed instead of the switch thereby to pick up a relay 132 having its coil connected in series with the normally open spring biased switch 131 between the busses 118. Holding contacts 132a thereof are connected in parallel with the switch 131 to keep the relay 132 picked up while the switch 131 is open. A set of normally open contacts 13212 are connected in parallel with the switch 120 so that when the timer motor 121 times out a complete cycle and closes the contacts 1223, the timer motor 121 remains energized and the cycle is automatically repeated.

The motor 100 which reciprocates the feeding mechanism 70 is serially connected with a switch 134 across the busses 118 so as to be continuously operated during the operation of the machine.

It will be apparent that the bar heating period and thus the bar temperature is primarily determined by the timer motor 119 which controls the timed ,bar heating interval substantially inversely proportional to the RMS value of the line voltage whereby the temperature of the heated bars 51 leaving the machine is substantially constant irrespective of changes in line voltage. To this end, the motor 119 is a DC motor connected in series with a diode 135 between the busses 118 thereby to be energized during alternate one-half cycles of the power source. Since the motor armature continues to rotate during the other half cycles a second diode 136 is connected across the motor to minimize the load on the motor. A conventional regulating circuit including a Zener diode 137, a capacitor 138 and a resistor 139 may, as shown, be connected in series with the timer motor 119. The diode 137 and the capacitor may be omitted.

While the present invention has been described in connection with particular embodiments thereof it will be understood that many changes and modifications may be made without departing from the true spirit and the scope of the present invention. It is intended, therefore, to cover all such changes and modifications in the appended claims.

What is claimed is:

1. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon,

means for connecting a source of electric current to said pairs of electrodes;

first and second-pairs of electrode supportsrespectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar, and

force applying means for moving the electrodes in each pair toward and away from one another,

one electrode support in each of said pairs including first and second members pivotally connected together with one of said clamping electrodes mounted to said first member, and

spring means interposed between said first and second members to store energy as said metal bar expands in length during heating and to exert a lateral force on said electrodes duringretraction of said supports away from said bar at the end of the heating cycle.

2. The combination according to claim '1 wherein said force applying means moves said first andsecnd members in a rectilinear direction along an axis substantially perpendicular to said bar,

stop means carried by one of said membersfor limiting the pivotal movement of said first member to a position on said axis, and

said spring means biases said firstmember against said stop means.

3. The combination set forth in claim 1 wherein said control means for supplying electric current to said electrodes and said bar comprises a DC timer motor for timing out the principal period during which said electric current is supplied to said electrodes and to said bar, and

rectifier means for supplying rectified AC voltage to said motor from the same source which supplies said electric current to said electrodes and to said bar.

4. The combination set forth in claim 1 wherein said source of electric current comprises a first power transformer connected to one electrode in each of said pairs of electrodes, and

a second power transformer connected to the other electrodes in each of said pairs of electrodes.

5. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon, means for connecting a source of electric current to said pairs of electrodes,

first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar,

force applying means for moving said electrode sup ports towards and away from said bar,

the electrode supports in at least one pair including spring means resiliently permitting the associated electrode to move away from the corresponding *10 electrodein the other. of saidipairs asthe bar increases in length 'duringheating, and control means for causing said'force applying means to move said clamping electrodes'against' said bar at a first clampingpressure,'forthen supplying electric current to said electrodes and through said bar to heat saidbar in the vicinity-of said electrodes, and for then substantially increasing the clamping force on said? bar. 6.ln apparatus for electrically heating a metal bar by passing electric current therethroughnthe combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart'locations thereon,

means for connectingasource of electric current to said pairs of electrodes,

first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar,

force applying means for moving said electrode supports towards and away from said bar,

the electrode supports in atleast one pair including spring means resiliently permitting the associated electrode to move away from the corresponding electrode in the other of said pairs as the bar increases in length during heating, and

said means for connecting a source of electric current to said pairs of electrodes comprises a plurality of elongated rigid conductors disposed in parallel relationship with said bar and respectively connected tosaid electrodes,

closely'spaced pairs of electric current supply conductors respectively connected between said rigid conductors near adjacent ends'thereof, and

said electrodes being connected to said rigid conductors at adjustable positions along said rigid conductors,

whereby said current supply conductors have asubstantially fixed spacing relative to the bar after said bar is clamped irrespective of the length or crosssection of thebar being heated.

7. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon,

means for connecting a source of electric current to said pairs of electrodes,

first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar,

force applying means formoving saidelectrode supports towards and away 'from said bar,

the electrode supports in at least one pair including spring means resiliently permitting the associated electrode to move away from the corresponding electrode in the other of said pairs as the bar increases in length during heating, and

means for feeding a plurality of said barsone-by-one to a position between said clamping electrodes, and

means for reciprocating said feeding means in a direction transverse to the direction of movement of said electrodes toward and away from one another,

whereby succeeding ones of said bars are engaged by substantially different surface areas of said electrodes.

8. Apparatus for electrically heating metal bars by passing electric current therethrough and including current supply means adapted to be connected to a source of AC voltage for supplying said electric current and timer means for controlling the length of time said current is passed through said bar, the improvement wherein said timer means, comprises a DC motor driven timer, and

a rectifier connected to said source of AC voltage and connected to said DC motor to supply rectified AC voltage thereto whereby the speed of said motor is substantially proportional to the RMS value of said electric current. 9. Apparatus according to claim 8 further comprising said current is initially passed through said bar. 

1. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon, means for connecting a source of electric current to said pairs of electrodes; first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar, and force applying means for moving the electrodes in each pair toward and away from one another, one electrode support in each of said pairs including first and second members pivotally connected together with one of said clamping electrodes mounted to said first member, and spring means interposed between said first and second members to store energy as said metal bar expands in length during heating and to exert a lateral force on said electrodes during retraction of said supports away from said bar at the end of the heating cycle.
 2. The combination according to claim 1 wherein said force applying means moves said first and second members in a rectilinear direction along an axis substantially perpendicular to said bar, stop means carried by one of said members for limiting the pivotal movement of said first member to a position on said axis, and said spring means biases said first member against said stop means.
 3. The combination set forth in claim 1 wherein said control means for supplying electric current to said electrodes and said bar comprises a DC timer motor for timing out the principal period during which said electric current is supplied to said electrodes and to said bar, and rectifier means for supplying rectified AC voltage to said motor from the same source which supplies said electric current to said electrodes and to said bar.
 4. The combination set forth in claim 1 wherein said source of electric current comprises a first power transformer connected to one electrode in each of said pairs of electrodes, and a second power transformer connected to the other electrodes in each of said pairs of electrodes.
 5. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon, means for connecting a source of electric current to said pairs of electrodes, first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar, force applying means for moving said electrode supports towards and away from said bar, the electrode supports in at least one pair including spring means resiliently permitting the associated electrode to move away from the corresponding electrode in the other of said pairs as the bar increases in length during heating, and control means for causing said force applying means to move said clamping electrodes against said bar at a first clamping pressure, for then supplying electric current to said electrodes and through said bar to heat said bar in the vicinity of said electrodes, and for then substantially increasing the clamping force on said bar.
 6. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon, means for connecting a source of electric current to said pairs of electrodes, first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutuAl alignment on opposite sides of said bar, force applying means for moving said electrode supports towards and away from said bar, the electrode supports in at least one pair including spring means resiliently permitting the associated electrode to move away from the corresponding electrode in the other of said pairs as the bar increases in length during heating, and said means for connecting a source of electric current to said pairs of electrodes comprises a plurality of elongated rigid conductors disposed in parallel relationship with said bar and respectively connected to said electrodes, closely spaced pairs of electric current supply conductors respectively connected between said rigid conductors near adjacent ends thereof, and said electrodes being connected to said rigid conductors at adjustable positions along said rigid conductors, whereby said current supply conductors have a substantially fixed spacing relative to the bar after said bar is clamped irrespective of the length or cross-section of the bar being heated.
 7. In apparatus for electrically heating a metal bar by passing electric current therethrough, the combination comprising first and second pairs of clamping electrodes for clampingly engaging said bar at spaced apart locations thereon, means for connecting a source of electric current to said pairs of electrodes, first and second pairs of electrode supports respectively supporting the electrodes in each of said pairs in substantial mutual alignment on opposite sides of said bar, force applying means for moving said electrode supports towards and away from said bar, the electrode supports in at least one pair including spring means resiliently permitting the associated electrode to move away from the corresponding electrode in the other of said pairs as the bar increases in length during heating, and means for feeding a plurality of said bars one-by-one to a position between said clamping electrodes, and means for reciprocating said feeding means in a direction transverse to the direction of movement of said electrodes toward and away from one another, whereby succeeding ones of said bars are engaged by substantially different surface areas of said electrodes.
 8. Apparatus for electrically heating metal bars by passing electric current therethrough and including current supply means adapted to be connected to a source of AC voltage for supplying said electric current and timer means for controlling the length of time said current is passed through said bar, the improvement wherein said timer means, comprises a DC motor driven timer, and a rectifier connected to said source of AC voltage and connected to said DC motor to supply rectified AC voltage thereto whereby the speed of said motor is substantially proportional to the RMS value of said electric current.
 9. Apparatus according to claim 8 further comprising a Zener diode connected in series with said DC motor and said rectifier across said source of AC voltage.
 10. Apparatus for electrically heating a metal bar by clampingly engaging said bar at spaced locations with sets of electrodes and passing electric current through said electrodes and said bar, the improvement comprising power means for initially moving said electrodes into engagement with said bar at a first pressure, and means for substantially increasing said pressure after said current is initially passed through said bar. 